1. Field of the Invention
The present invention relates to hydraulic valve lefters in a valve operating mechanism of an internal combustion engine, wherein cams work directly on valve tappets.
2. Description of the Related Art
In conventional hydraulic valve lifters which are known from U.S. Pat. No. 4,424,774 and U.S. Pat. No. 4,530,320, cams are situated closely to the upper ends of corresponding valve stems to provide a low profile valve operating mechanism. FIG. 3 illustrates a hydraulic valve lifter 31, as an example of such a conventional hydraulic valve lifter, which includes two cup like cam followers or tappets. More specifically, an inverted cup like body 34 has a cylindrical skirt 32 slidably fit within a guide hole 11 of the cylinder head I and a disk shaped tip 33 adapted to contact the cam 9. An inverted cup-like plunger 37 has a side wall 35 slidable axially within the skirt 32 of the body 34 and an end wall 36 confronting the tip 33 of the body 34. A high pressure chamber 88 is defined between the tip 33 of the body 34 and the end wall 36 confronting the tip 33. A oil is fed from the feed hole which is defined in the guide hole 11 of the cylinder head 1. The body 34 and the plunger 37 together form an oil passage 39 through which the oil is fed to the high pressure chamber 38. The plunger 37 slides downwardly and is pressed against the upper end of a valve stem 4 by hydraulic pressure of the oil 40 filled in the high pressure chamber 38.
The oil fed to the hydraulic valve lifter 31 tends to flow between a guide hole 11 of a cylinder head 1 and the skirt 32, and between the skirt 32 of the body and the side wall 35 of the plunger and is thereafter directed mostly to the outside of a valve spring 8 surrounding the valve stem 4 rather than to the valve stem 4 within the valve spring 8. As a result, the valve stem 4 and a tubular valve guide 2 which is formed in the cylinder head 1 may be burnt when an internal combustion engine runs at extremely high speeds. Such burning may cause the valve 3 to stick and break. The valve breakage may further cause the hydraulic valve lifter, the cylinder and a piston to break. Repair of those broken components requires a considerable time and is costly.